在php中编写xml文件时收到xmlelement警告

我用以下代码在php中编写xml文件，但当我运行这些代码时，我会得到以下错误

SimpleXMLElement::asXML(): string is not in UTF-8

生成xml的代码如下

<?php
    //Create Database connection
  $mysqli = new mysqli('localhost', 'user', 'pass', 'dbnam');
   if(mysqli_connect_errno()) {
      echo "Connection Failed: " . mysqli_connect_errno();
      }
 $query='select Siteurl from tablename order by colname2 desc ';
    $result = mysqli_query($mysqli,$query);  
    //Create SimpleXMLElement object
    $xml = new SimpleXMLElement('<urls/>');
    //Add each column value a node of the XML object
    while($row = mysqli_fetch_array($result)) {
        $mydata = $xml->addChild('url');
        $mydata->loc=$row['Siteurl'];
    }
  mysqli_close($mysqli);
    //Create the XML file
    $fp = fopen("sitemaps/sitemap2.xml","wb");
    //Write the XML nodes
    fwrite($fp,$xml->asXML());
    //Close the database connection
    fclose($fp);
?>

伙计们，我该怎么纠正错误呢。请帮助我